Various emissions from motor-vehicle exhaust systems are regulated, such as NOx emission. To limit NOx emission while maintaining performance, a motor-vehicle may be configured, in a closed-loop manner, to tighten emission control when an exhaust-stream NOx level increases. Tightening control may include, for example, supplying a richer air/fuel mixture to a combustion chamber of the motor vehicle or enabling reductive regeneration of an exhaust-stream NOx trap.
For some control systems to function effectively, an accurate estimate of the NOx level may be generated from a NOx sensor. U.S. Pat. No. 4,770,760 describes a NOx sensor comprising two electrochemical pump cells in series, with a diffusion restrictor upstream of each pump cell. In the cited reference, the NOx level in an analyte is indicated by a diffusion-limited current flowing to a catalyzed electrode in a downstream pump cell, an upstream pump cell being used to reduce the partial pressure of oxygen (O2) to a fixed, low level, so that the NOx level can be estimated with a minimum of interference.
In the decade since this technology was developed, numerous attempts have been made to improve the accuracy, reliability, and longevity of electrochemical NOx sensors. For example, U.S. Pat. No. 6,059,947 describes a NOx sensor providing feedback control of a set-point voltage in the upstream pump cell, and a self-diagnosis feature coupled to the feedback control. The feedback control adjusts the set-point voltage so that the partial pressure of O2 in the second pump cell remains constant despite aging of sensor components and changes in analyte O2 level. The self-diagnosis feature compares the adjusted set point against predetermined limits to assess sensor degradation.
However, the above reference fails to address other ageing effects commonly observed in electrochemical NOx-sensor response: namely, a gradual reduction in gain and an increase in offset in the correlation between the sensed parameters and NOx in the exhaust stream. These factors, which contribute to uncertainty in the detected NOx level, may be caused by degradation losses occurring on prolonged operation of the NOx sensor: losses in electrolyte conductance, for example, or in electroactivity and/or electroactive surface area of one or more electrodes.
The inventors herein have recognized the above problems and have devised a series of approaches to address them. Thus, in one embodiment, a system for calibrating a response to an exhaust-stream NOx level in a motor vehicle is provided. The system comprises a NOx sensor that includes an electrode, a current from the electrode responsive to the exhaust-stream NOx level while a bias voltage is applied to the electrode. The system further comprises a controller configured to interrupt the bias voltage and to adjust a motor-vehicle response to the current based at least partly on an attained voltage of the electrode while the bias voltage is interrupted.
By calibrating the motor-vehicle response based on the attained voltage, the motor-vehicle response may track the exhaust-stream NOx level with greater fidelity despite degradation losses in the NOx sensor, such as those indicated above.
Other embodiments disclosed herein provide a method of calibrating a response to an exhaust-stream NOx level in a motor vehicle. Still other embodiments provide a method of calibrating the NOx sensor response in terms of gain and offset parameters.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.